DISRUPTION OF THE AVIRULENCE GENE AVR9 IN 2 RACES OF THE TOMATO PATHOGEN CLADOSPORIUM-FULVUM CAUSES VIRULENCE ON TOMATO GENOTYPES WITH THE COMPLEMENTARY RESISTANCE GENE CF9

To study the function of the avirulence gene avr9 of the tomato pathog en Cladosporium fulvum, we developed procedures for gene disruption ex periments in two different races of the fungus both avirulent on tomat o genotypes carrying the resistance gene Cf9. For this purpose we sele cted uridine auxotrophic strains amongst fluoroorotic acid resistant m utants. These mutants were transformed with a plasmid containing the a vr9 genomic region in which the open reading frame was replaced by the pyrG gene from Aspergillus nidulans. For each of the two races used w e selected one transformant in which the entire avr9 coding sequence w as deleted as a result of a gene replacement event. The two transforma nts were able to successfully infect Cf9 tomato genotype, unlike their wild-type avr9+ parents, which induced hypersensitive responses on th is genotype. We also demonstrated that these two transformants no long er produce the necrosis-inducing elicitor peptide specifically interac ting with Cf9 tomato plants. These results confirm that the cloned avr 9 sequence (and therefore the AVR9 peptide) is fully responsible for a virulence in wild-type avr9+ races of the fungus. These results also i ndicate that this gene is dispensible for normal vegetative growth and pathogenicity at least in a monocyclic process. These results are dis cussed in relation to the possible origin of avr9- strains and the lon g-standing resistance toward C. fulvum offered by the tomato Cf9 resis tance gene.